This invention relates to an electron beam butt welding process performed at pressures of 10 millibars and more, especially at atmospheric pressure.
Electron beam welding at gas pressures of from 10 millibars to standard pressure and over, in which the electron beam is produced in a vacuum and passes into a region of higher pressure or into the free atmosphere, that is, electron beam welding in which the workpieces are situated in conditions wherein the air pressure is higher than this vacuum, is already known. Due to the inevitable collisions which take place between electrons and gas molecules, however, the electron beam becomes divergent after traveling a distance of only a few millimeters, so that its power density, with respect to the available beam cross section, is reduced proportionally. Consequently, the known method has been limited to the welding of relatively thin workpieces where the distance between the workpiece and the aperture through which the beam emerges from the electron gun is short.
In electron beam welding, vaporization and spattering of the molten workpiece takes place, resulting in an undesirable depositing of material on the gun orifice, which can also be called a nozzle. To prevent contamination of the nozzle by the welding process itself, the weld site must be no closer than a certain minimum distance away from the aperture through which the electron beam emerges from the electron gun. In conflict with this requirement is the fact that this distance can be no greater than some 25 millimeters because the electron beam, which emerges from the electron gun well focused and with a very high power density, is so severely dissipated along this distance to the workpiece, for the reasons stated above, that its power density drops to a level which is no longer sufficient to perform the deep welds which are possible in electron beam welding under vacuum.
Whereas in the case of high-vacuum electron beam welding, the depth-to-width ratio of the weld can be as great as 50:1, it is not possible normally to achieve depth-to-width ratios of more than 4:1 with an electron beam that enters the atmosphere.
Electron beam welding under atmospheric conditions has the considerable advantage that large and/or complexly shaped workpieces can be welded without the need for a vacuum chamber of the size needed to accommodate them. This relates especially to the longitudinal welding of pipes and the butt welding of individual sections of pipe. In order to be able to dispense with large-capacity vacuum chambers in welding such workpieces, it is also known to use electron beam guns which are sealed off against the workpiece surface by means of a sealing strip or an especially shaped cascade pressure frame, so that a chamber of limited size is created for the establishment of a sufficiently good vacuum. However, such a solution of the problem is complex and must be tailored to the workpiece dimensions and shapes, so that this method has not achieved any appreciable acceptance thus far (German Auslegeschrift No. 1,515,201).